How to take an action on startup

A global setting onLoad is of type State => State and is executed once, after all projects are built and loaded. There is a similar hook onUnload for when a project is unloaded.

Project unloading typically occurs as a result of a reload command or a set command. Because the onLoad and onUnload hooks are global, modifying this setting typically involves composing a new function with the previous value. The following example shows the basic structure of defining onLoad.

Suppose you want to run a task named dependencyUpdates on start up. Here’s what you can do:

lazy val dependencyUpdates = taskKey[Unit]("foo")
// This prepends the String you would type into the shell
lazy val startupTransition: State => State = { s: State =>
"dependencyUpdates" :: s
}
lazy val root = (project in file("."))
.settings(
scalaVersion in ThisBuild := "2.12.3",
organization in ThisBuild := "com.example",
name := "helloworld",
dependencyUpdates := { println("hi") },
// onLoad is scoped to Global because there's only one.
onLoad in Global := {
val old = (onLoad in Global).value
// compose the new transition on top of the existing one
// in case your plugins are using this hook.
startupTransition compose old
}
)